Braking and controllability control method and system for a vehicle with regenerative braking

ABSTRACT

The present invention provides a method and system to use feedback control algorithms to monitor and dynamically modify front and rear braking torque to initiate braking based on driver demand, initially favoring regenerative braking more than conventional braking balance would indicate while monitoring and maintaining vehicle controllability factors such as oversteer and understeer. A simple proportional-integral-derivative feedback controller can be used. Vehicle sensors for wheel speed, lateral acceleration, yaw rate, and brake position can provide input to the controller to monitor vehicle conditions and to activate non-regenerative and regenerative braking in varying proportions based on at least one actual vehicle controllability value and predetermined target value for controllability and optimization of energy recovery. Controllability factors can include predetermined longitudinal wheel slip ratios or a comparison of tire slip angle or yaw rate to a target value.

FIELD OF INVENTION

The present invention relates generally to vehicle braking andcontrollability control systems, commonly referred to as stabilitycontrol systems, and specifically to a braking and controllabilitycontrol method and system for a vehicle with regenerative braking thatoptimizes energy recovery while reducing vehicle oversteer andundersteer.

BACKGROUND OF THE INVENTION

The need to reduce fossil fuel consumption and emissions in automobilesand other vehicles predominately powered by internal combustion engines(ICEs) is well known. Vehicles powered by electric motors attempt toaddress these needs. Another alternative solution is to combine asmaller ICE with electric motors into one vehicle. Such vehicles combinethe advantages of an ICE vehicle and an electric vehicle and aretypically called hybrid electric vehicles (HEVs). See generally, U.S.Pat. No. 5,343,970 to Severinsky.

The HEV is described in a variety of configurations. Many HEV patentsdisclose systems where an operator is required to select betweenelectric and internal combustion operation. In other configurations, theelectric motor drives one set of wheels and the ICE drives a differentset.

Other, more useful, configurations have developed. For example, a serieshybrid electric vehicle (SHEV) configuration is a vehicle with an engine(most typically an ICE) connected to an electric motor called agenerator. The generator, in turn, provides electricity to a battery andanother motor, called a traction motor. In the SHEV, the traction motoris the sole source of wheel torque. There is no mechanical connectionbetween the engine and the drive wheels. A parallel hybrid electricalvehicle (PHEV) configuration has an engine (most typically an ICE) andan electric motor that work together in varying degrees to provide thenecessary wheel torque to drive the vehicle. Additionally, in the PHEVconfiguration, the motor can be used as a generator to charge thebattery from the power produced by the ICE.

A parallel/series hybrid electric vehicle (PSHEV) has characteristics ofboth PHEV and SHEV configurations and is sometimes referred to as a“split” parallel/series configuration. In one of several types of PSHEVconfigurations, the ICE is mechanically coupled to two electric motorsin a planetary gear-set transaxle. A first electric motor, thegenerator, is connected to a sun gear. The ICE is connected to a carriergear. A second electric motor, a traction motor, is connected to a ring(output) gear via additional gearing in a transaxle. Engine torque canpower the generator to charge the battery. The generator can alsocontribute to the necessary wheel (output shaft) torque if the systemhas a one-way clutch. The traction motor is used to contribute wheeltorque and to recover braking energy to charge the battery. In thisconfiguration, the generator can selectively provide a reaction torquethat may be used to control engine speed. In fact, the engine, generatormotor and traction motor can provide a continuous variable transmission(CVT) effect. Further, the HEV presents an opportunity to better controlengine idle speed over conventional vehicles by using the generator tocontrol engine speed.

The desirability of combining an ICE with electric motors is clear.There is great potential for reducing vehicle fuel consumption andemissions with no appreciable loss of vehicle performance ordriveability. The HEV allows the use of smaller engines, regenerativebraking, electric boost, and even operating the vehicle with the engineshutdown. Nevertheless, new ways must be developed to optimize the HEV'spotential benefits.

One such area of HEV development is optimizing the braking and stabilitysystem of the HEV or any other type of vehicle using regenerativebraking technology. Regenerative braking (regen) captures the kineticenergy of the vehicle as it decelerates. In conventional vehicles,kinetic energy usually dissipates as heat in a vehicle's brakes orengine during deceleration. Regen converts the captured kinetic energythrough a generator into electrical energy in the form of a storedcharge in a vehicle's battery. This stored energy is later used to powerthe electric motor. Consequently, regen also reduces fuel usage andemission production. In certain vehicle configurations, the engine canbe disconnected from the rest of the powertrain thereby allowing more ofthe kinetic energy to be converted into stored electrical energy.

On most vehicles with regenerative braking, the regenerative brakingtorque is applied to, or predominantly to, the wheels of only one axle.When regenerative braking is applied to the wheels of only one axle,non-regenerative braking methods may be used at the wheels of the otheraxles. The desire to recover energy through regenerative braking canresult in different braking torques being applied to the wheels of thedifferent axles. The difference between the braking torques can causeunbalanced braking that may degrade vehicle controllability. Degradedcontrollability can be in the form of either reduced stability orreduced steerability. For example, when excessive regenerative brakingtorque is applied at the front axle, as in front wheel drive vehicles,the ability of the front wheels to steer the vehicle may be reduced. Thereduced steerability is a condition known as understeer. When excessiveregenerative braking torque is applied at the rear axle, as in rearwheel drive vehicles, the lateral friction of the rear tires may bereduced. The reduced stability is a condition known as oversteer. Bothof these effects, understeer due to excessive levels of regenerativebraking at the front axle and oversteer due to excessive levels ofregenerative braking at the rear axle, can become greater on lowfriction surfaces such as ice and snow. The requirement forcontrollability on low friction surfaces typically forces regenerativebraking levels to be reduced, resulting in a corresponding loss ofenergy recovery.

There are HEV patents directed to control of regenerative brakingfunctions in various driving conditions. Koga et al. (U.S. Pat. No.6,033,041) describes a regenerative braking control system for anelectric vehicle where the regenerative braking varies as a function ofvehicle inclination. Okamatsu (U. S. Pat. No. 4,335,337) describes acontrol system for an electric powered vehicle. This invention attemptsto improve tire grip performance by adjusting the frequency of therotations of the induction motors based on the slip frequency of thevehicle without regard to regenerative braking.

Ohtsu et al., (U.S. Pat. No. 5,476,310) also attempts to improve brakingperformance through the cooperation of mechanical anti-lock brakes andregenerative braking. This invention regulates excessive braking forceand slip with a controller using a predetermined slip ratio. Otherinventions also attempt to regulate excessive slip. See Asa et al. (U.S.Pat. No. 5,654,887) and Kidston et al. (U.S. Pat. No. 5,615,933).Unfortunately, while these inventions do reduce excessive slip, they donot provide an adequate level of stability because they focus mainly onthe maximization of straight line stopping.

Asanuma et al. (U.S. Pat. No. 5,318,355), describes a switchover modefrom a regenerative or friction braking mode of operation.Unfortunately, this invention is susceptible to false activation of themode switchover.

Thus the ability to distribute brake torque between regenerative andnon-regenerative braking while optimizing energy recovery and vehiclecontrollability constitutes and unmet need in the art.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method and system tocontrol braking and improve controllability of a vehicle equipped withregenerative braking. The invention can provide regenerative brakingwhile reducing understeer and oversteer while not significantly reducingenergy recovery, even on low friction surfaces. The inventioncontinuously detects oversteer and understeer and correspondinglybalances regenerative braking torque with conventional non-regenerativebraking torque if controllability decreases.

In accordance with an important aspect of the present inventionregenerative braking torque is applied to the wheels of at least oneaxle, mechanical friction or other non-regenerative brakes known in theart being connected to the wheels of another axle. In accordance withthe invention the regenerative braking and non-regenerative brakingforces are applied to the wheel of different axles. The vehicle has acontroller having the microprocessor hardware and software to receiveand evaluate sensor input of brake position and wheel speed of eachwheel and to activate a generator motor which varies non-regenerativeand regenerative braking in optimum proportions between the front axleand rear axle for maximum energy recovery and vehicle controllability.Vehicle controllability is determined based on at least one actualvehicle controllability value and at least one predetermined targetvalue. Regenerative braking is adjusted so as to maintain the actualvehicle controllability value within the predetermined target value. Thecontroller can be a simple proportional-integral-derivative feedbacktype of controller.

The present invention can reduce oversteer in vehicle configurationswhere the generator motor provides variable amounts of regenerativebraking torque to the wheels of the rear axle and where thenon-regenerative brakes are connected to the wheels of the front axle.The sensor input preferably includes data related to steering angle, yawrate and lateral acceleration. In addition, vehicle controllabilitydeterminations, such as oversteer, can be based on data related tolongitudinal wheel slip ratio, tire slip angle, and yaw rate. Steeringangle can be determined from steering wheel position, steerable wheelposition or a time-filtered determination of steering angle. The presentinvention can also be configured to reduce understeer in front wheeldrive vehicles.

Other objects and features of the present invention will become moreapparent to persons having ordinary skill in the art to which thepresent invention pertains from the following description and claimstaken in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing objects, advantages, and features, as well as otherobjects and advantages, will become apparent with reference to thedescription and figures below, in which like numerals represent likeelements and in which:

FIG. 1 illustrates a general hybrid electric vehicle (HEV)configuration.

FIG. 2 illustrates a braking and controllability control strategy of thepresent invention.

DETAILED DESCRIPTION

The present invention relates to electrically propelled vehicles such aselectric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cellelectric vehicles (FCEVs) that have a regenerative braking system. Thepresent invention is a system to continuously control braking andcontrollability for a vehicle with regenerative braking. FIG. 1demonstrates just one possible configuration, specifically aparallel/series hybrid electric vehicle (split) configuration.

In a basic HEV, a planetary gear set 20 mechanically couples a carriergear 22 to an engine 24 via a one-way clutch 26. The planetary gear set20 also mechanically couples a sun gear 28 to a generator motor 30 and aring (output) gear 32. The generator motor 30 also mechanically links toa generator brake 34 and is electrically linked to a battery 36. Atraction motor 38 is mechanically coupled to the ring gear 32 of theplanetary gear set 20 via a second gear set 40 and is electricallylinked to the battery 36. The ring gear 32 of the planetary gear set 20and the traction motor 38 are mechanically coupled to drive wheels 42via an output shaft 44 that is mechanically coupled to a rear axle 66having the drive wheels 42. For purposes of illustration, the vehiclecan also have a separate pair of non-driven steerable wheels 64connected by a front axle 68. The non-driven steerable wheels 64 arepositioned toward the front of the vehicle while the drive wheels 42 arepositioned toward the rear of the vehicle.

The planetary gear set 20, splits the engine 24 output energy into aseries path from the engine 24 to the generator motor 30 and a parallelpath from the engine 24 to the drive wheels 42. Engine 24 speed (or RPM)can be controlled by varying the split to the series path whilemaintaining the mechanical connection through the parallel path. Thetraction motor 38 augments the engine 24 power to the drive wheels 42 onthe parallel path through the second gear set 40. The traction motor 38also provides the opportunity to use energy directly from the seriespath, essentially running off power created by the generator motor 30.This reduces losses associated with converting energy into and out ofchemical energy in the battery 36 and allows all engine 24 energy, minusconversion losses, to reach the drive wheels 42.

A vehicle system controller (VSC) 46 controls many components in thisHEV configuration by connecting to each component's controller. Anengine control unit (ECU) 48 can connect to the engine 24 via a hardwireinterface. All vehicle controllers can be physically combined in anycombination or can stand as separate units. They are described asseparate units here because they each have distinct functionality. Thevehicle controllers have the microprocessor hardware and software toreceive and evaluate sensor input and respond according to that input.The VSC 46 communicates with the ECU 48, as well as a battery controlunit (BCU) 50 and a transaxle management unit (TMU) 52 through acommunication network such as a controller area network (CAN) 54. Inthis illustration, the BCU 50 connects to the battery 36 via a hardwireinterface. The TMU 52 controls the generator motor 30 and traction motor38 via a hardwire interface.

Further, the VSC 46 can communicate with an electric hydraulic brakingunit (EHBU) 56 through the CAN 54. The EHBU 56 is connected tonon-regenerative brakes 58 (e.g., mechanical brakes) that ultimately areconnected to the non-driven steerable wheels 64. The EHBU 56 can controlanti-lock braking systems (ABS), regenerative braking, traction control,and non-regenerative braking. In the present invention, braking controlfor each drive wheel 42 and non-driven steerable wheel 64 isindependently available.

The VSC 46 can receive input from various vehicle component sensors. Inaccordance with an embodiment of the present invention inputs areprovided by: a brake position sensor 62 (such as a brake pedal), wheelspeed sensors 70, steerable wheel position sensors 72, and inertialsensors 74. The inertial sensors 74 can measure, for example, lateralacceleration and/or yaw rate. The brake position sensor 62 output can besent to the VSC 46 or the EHBU 56 depending on vehicle configuration.For the present illustration, the brake position sensor 62 output issent to the EHBU 56. Wheel speed sensors 70 are located at each vehiclewheel. The present invention provides a method and system forcontinuously adjusting braking between front and rear axles to optimizeregenerative energy recovery while maintaining vehicle controllability.A control system for a vehicle equipped with regenerative braking, inone possible configuration, is illustrated in FIG. 1. The invention canprovide regenerative braking while reducing vehicle understeer andoversteer even on low friction surfaces while not significantly reducingenergy recovery on high friction surfaces.

The controller of the present invention can be physically located eitherwithin the VSC 46 or as a stand-alone unit, such as the EHBU 56. Thecontroller continuously monitors vehicle controllability andcorrespondingly commands a change in regenerative braking in real timewhenever vehicle controllability is reduced. The system determinesvehicle controllability based on at least one predetermined targetvalue.

As stated above, most vehicle configurations with regenerative brakingapply braking torque to the wheels of one axle (or predominately to thewheels of one axle). When regenerative braking is applied to the wheelsof only one axle, conventional non-regenerative braking methods can beused at the wheels of the other axles to balance the vehicle's overallbraking torque. To demonstrate this in the vehicle configuration in FIG.1, the EHBU 56 could command activation of regenerative braking at thedrive wheels 42 of the rear axle 66 (i.e., a rear wheel driveconfiguration). The EHBU 56 could concurrently command an application ofthe conventional non-regenerative brakes 58 to the non-driven steerablewheels 64 of the front axle 68. In this example optimal regenerativeenergy is not realized because any braking torque using conventionalnon-regenerative brakes 58 results in kinetic energy wasted as heat.Ideally, maximum energy recovery would occur with complete regenerativebraking.

A challenge to obtaining maximum energy recovery through regenerativebraking is due to unbalanced braking torques between the front and rearwheels, which can affect vehicle controllability. For example, in afront wheel drive vehicle (not shown), if excessive braking torque isapplied to the front steerable wheels of the front axle in an attempt tomaximize energy recovery (i.e., less conventional non-regenerativebraking force is applied to the wheels on the rear axle), the ability tosteer the steerable wheels 64 is reduced (understeer). In a rear wheeldrive vehicle, when excessive braking torque is applied at the drivewheels 42 of the rear axle 66 in an attempt to maximize energy recovery(e.g., less conventional non-regenerative braking force is applied tothe front non-driven steerable wheels 64), the lateral friction of thedrive wheels 42 is reduced (oversteer). These controllability problemscan become more severe on low friction surfaces such as ice and snow.The present invention thus provides a method and apparatus formaximizing energy recovery while maintaining vehicle controllability.

During a braking event, the present invention will initially rely onregenerative braking, while continuously monitoring vehiclecontrollability and adjust braking accordingly. The system attempts tocorrect control problems by maintaining vehicle controllability within apredetermined target value. This means the system of the presentinvention should react before conventional vehicle stability controlsystems engage.

To achieve the goal of vehicle controllability, objectives of thepresent invention are to monitor the development of any lateralinstability (such as oversteer or high longitudinal wheel slip ratio)and reduce regenerative braking to achieve a more conservative brakebalance. Longitudinal wheel slip ratio is determined by measuring thespeed of the front wheels and the speed of the rear wheels using wheelspeed sensors 70. A formula for longitudinal wheel slip ratio is:${{Longitudinal}\quad {Weel}\quad {Slip}\quad {Ratio}} = {1 - \frac{{{Vehicle}\quad {Speed}} - {{Wheel}\quad {Speed}}}{{Vehicle}\quad {Speed}}}$

As a first approximation of vehicle speed, the wheel speed of the frontwheels can be used for a rear wheel drive vehicle and the wheel speed ofthe rear wheels can be used for a front wheel drive vehicle. When rearwheel brakes are applied, vehicle deceleration comes from the rearwheels. For example, a ten percent (10%) longitudinal wheel slip ratiomeans the rear wheels are traveling ten (10%) slower than the vehicle.As the longitudinal wheel slip ratio increases for the rear wheels,lateral stability decreases.

The present invention uses feedback control algorithms to monitor anddynamically modify relative front and rear braking torque to initiatebraking based on driver demand, initially favoring regenerative brakingmore than conventional braking balance would indicate, while monitoringand maintaining vehicle controllability. For a rear wheel drive vehicle,oversteer can be reduced and for a front wheel drive vehicle, understeercan be reduced.

FIG. 2 illustrates one possible configuration of the present inventionby way of a block diagram of a regenerative braking controller utilizingfeatures of the present invention. As stated above, this controller canbe housed within the VSC 46 or a separate controller, such as the EHBU56. This controller can generally include a simpleproportional-integral-derivative feedback controller. The strategyillustrated in FIG. 2 can be used for a two-wheel drive, rear wheeldrive vehicle with regenerative braking applied, or predominatelyapplied, to the rear wheels to reduce oversteer situations. It is noted,though, that a front wheel drive vehicle configuration can also use thepresent invention to reduce understeer. For determination andalleviation of vehicle controllability problems such as oversteer andundersteer, the strategy can continuously monitor longitudinal wheelslip ratios, tire slip angle, and yaw rate individually or in anycombination. For purposes of illustration, the strategy presented inFIG. 2 utilizes all three methods to monitor vehicle oversteer andundersteer. The strategy of FIG. 2 can also continuously adjust theproportion of regenerative and non-regenerative braking to optimizeenergy recovery while maintaining the actual vehicle controllabilitywithin predetermined controllability target values.

In FIG. 2, the strategy starts with each “key-on” event and ends witheach “key-off” event. The strategy, once initiated at step 90, canmonitor several vehicle inputs from the wheel speed sensors 70,steerable wheel position sensors 72 and brake position sensor 62.

After step 90, the strategy proceeds to step 92 and determines whetherthe vehicle operator has demanded a steering angle deviating from deadcenter (i.e., a turn). In other words, the strategy determines thedegree of steering angle deviation left or right of a straight aheaddirection of travel by the vehicle. If no at step 92, the strategyproceeds to step 88 and sets target tire slip angle and yaw rate to “0”then proceeds to step 96. If yes at step 92, the strategy proceeds tostep 94. At step 94, the strategy calculates a target tire slip angleand target yaw rate (turn rate) and proceeds to step 96.

At step 96, the strategy determines whether a braking force has beencommanded. A braking force command can come from the vehicle operator orfrom the VSC 46. A braking force can be requested by the vehicleoperator where the braking force to be applied to the vehicle wheels isdetermined using input from the brake position sensor 62 and the vehicleapplies braking force in relation to the position of the brake positionsensor 62. A braking force can also be requested by the VSC 46, such asto simulate the engine braking during coast-down of a traditional ICEonly vehicle. If no, the strategy cycles back to step 90. If yes, thestrategy proceeds to step 98.

At step 98, the strategy commands braking torque using the availableregenerative braking to the extent possible. For the presentillustration, regenerative braking would be applied to the rear axledrive wheels 42. In an alternate embodiment having a front wheel driveconfiguration, the strategy would apply regenerative braking to thefront wheels. Next, the strategy proceeds to step 100 to determine:wheel speed for the drive wheels 42 on the rear axle 66 and thenon-driven steerable wheels 64 on the front axle 68; lateralacceleration; and yaw rate. These determinations can be obtained usingvarious vehicle inputs such as those obtained from wheel speed sensors70, and inertial sensors 74 of a type known in the art. The strategythen proceeds to step 102.

At step 102, the strategy calculates a longitudinal wheel slip ratiofrom the difference in wheel speeds and vehicle speed (as describedabove) and then proceeds to step 104 to make determinations of whethervehicle controllability indicators are within predetermined thresholdvalues.

At step 104, the strategy determines whether the calculated longitudinalwheel slip ratio of step 102 exceeds a predetermined longitudinal wheelslip ratio value. For this illustration, a longitudinal wheel slip ratiovalue of ten percent (10%) is used, but a preferred longitudinal wheelslip ratio is five percent (5%). The predetermined longitudinal wheelslip ratio value can also be a dynamic variable that is dependent onvehicle operating conditions.

If yes at step 104, the strategy proceeds to step 106 and commandsapplication of the non-regenerative brakes 58 on the wheels of the frontaxle 68 and the proportional reduction of regenerative braking on thewheels of the rear axle 66 to balance braking torque until the vehicleis within predetermined thresholds for controllability. Overall brakingforce remains the same or consistent with the commanded braking force.Only the proportion of regenerative braking is reduced. If no at step104, the strategy proceeds to step 108.

At step 108, the strategy determines whether actual tire slip angleexceeds the target tire slip angle. If yes, the strategy proceeds tostep 106; if no, the strategy proceeds to step 110.

At step 110, the strategy determines whether the actual yaw rate exceedsthe target yaw rate. If yes, the strategy proceeds to step 106; if no,the strategy cycles back to step 90.

As described in the strategy illustrated in FIG. 2, specificcalculations need to be developed to obtain target and actual values forlongitudinal wheel slip ratios, tire slip angle, or yaw rates topractice the present invention. For a first calculation, the formula forlongitudinal wheel slip ratio shown above can be used.

For a second calculation, tire slip angle is factored to determine amodified tire slip angle. The modified tire slip angle can also be usedto determine acceptable vehicle controllability by estimating an alpha,the tire slip angle, and its rate of change, determining a target valueof alpha from a tire slip angle determination, and using closed loopcontrol to reduce the level of regenerative braking to the requiredamount.

Alpha and its time derivative, alpha_dot, can be estimated by knownmethods of calculation using inertial sensors 74. Other methods known inthe art can also be used to infer tire slip angle such as GlobalPositioning System sensors (not shown), optical sensors (not shown),radar (not shown) and other like technologies. If the regenerativebraking torque were to be applied at the rear axle 66 wheels (orprimarily at the rear wheels) and therefore oversteer was the concern,then alpha and its derivative would be estimated for the rear axle 66wheels. If the regenerative braking torque were to be applied at thefront axle 68 wheels (or primarily at the front wheels) and thereforeundersteer was the concern, then alpha and its derivative would beestimated for the front axle 68 wheels. Note that since the front axle68 wheels are steerable, the calculation of the alpha estimate wouldalso include the steerable wheel position. That would also be true ofthe rear axle 66 wheels if they were steered.

Lateral acceleration, Ay, of the vehicle can be measured or estimated ata location corresponding to the location of alpha. The target value foralpha, alpha_target, is calculated in proportion to the absolute valueof Ay according to the following equation:

alpha_target=CC*abs(Ay)+alpha_offset,

where CC is the cornering compliance of the tires at the location of thealpha estimate and alpha_offset is a constant compensating forestimation error for alpha. The equation yields a value for alpha_targetthat is always positive. In the above equation, the measured value of Aycould be replaced by a value determined in some other manner thatrepresented the limit of lateral acceleration corresponding to availablesurface friction.

The closed loop control of tire slip angle, alpha, using regenerativebraking torque, T_regen, can be implemented as follows:

T_correction_r=max(kp*(alpha−alpha_target)+kd*alpha_dot, 0),

where kp and kd are calibratible values;

T_correction_l=max(−kp*(alpha+alpha_target)−kd*alpha_dot, 0);

T_correction=max(T_correction_r, T_correction_l); and

T_regen=T_desired_regen−T_correction.

The above equations are an implementation of a simpleproportional-integral-derivative feedback controller based on theassumption that T_regen and T_desired_regen are always positive values.The torque correction, T_correction_r, corresponds to a tire slip anglein one direction (a right hand turn) while T_correction_l corresponds tothe other direction. These equations will act to reduce the level ofregenerative braking torque when the tire slip angle exceeds the targetlevel.

For the third method, vehicle yaw rate, YR, is used as the indicator ofvehicle controllability. Acceptable vehicle controllability ismaintained by measuring the yaw rate, calculating a target value, andusing closed loop control to reduce the level of regenerative braking asrequired to ensure that the yaw rate does not exceed the target value.

Vehicle yaw rate, YR, would be a signed value. The target value of yawrate, YR_target, would be a signed value that is calculated using themethods which are well established in the practice of vehicle stabilitycontrol. The closed loop control of yaw rate, YR, would be implementedby reducing regenerative braking torque with a correction term,T_correction, as follows:

T_correction_r=max(kp*(YR−YR_target)+kd*YR_dot+ki*YR_int, 0);

T_correction_l=max(−kp*(YR−YR_target)−kd*YR_dot−ki*YR_int, 0);

T_correction=max(T_correction_r, T_correction_l); and

T_regen=T_desired_regen−T_correction,

where kp, kd and ki are calibratible values, YR_dot is the derivative ofYR or (YR−YR_target) and YR_int is the integral of (YR−YR_target).

The above equations are an implementation of a simpleproportional-integral-derivative feedback controller.

The above-described embodiments of the invention are provided purely forpurposes of example. Many other variations, modifications, andapplications of the invention may be made. Variations could include, butare not limited to, applying the invention to front wheel drivevehicles, rear wheel drive and all-wheel drive vehicles. Additionallyvariations could include, but are not limited to, applying the inventionto front wheel steer vehicles, rear wheel steer vehicles and all-wheelsteer vehicles.

What is claimed is:
 1. A system for controlling braking of a vehiclecomprising: regenerative brakes connected to wheels mounted on a firstaxle of said vehicle; non-regenerative brakes connected to wheelsmounted on a second axle of said vehicle different from said first axle;a plurality of sensors for measuring and providing electronic signals tomonitor vehicle inputs; a controller adapted to continuously receive andprocess said signals; and a generator motor activated by said controllerfor adjustably activating non-regenerative and regenerative braking invarying proportions between the wheels of said first axle and the wheelsof said second axle, thereby maintaining a vehicle controllability valuewithin a preselected target range.
 2. The system of claim 1, wherein theplurality of sensors comprise a brake pedal position and wheel speed ofeach wheel of said vehicle.
 3. The system of claim 1, wherein thecontroller is a simple proportional-integral-derivative feedbackcontroller.
 4. The system of claim 2, wherein: the generator motorprovides regenerative braking torque to wheels of a rear axle; thenon-regenerative brakes are connected to wheels of a front axle; and theplurality of sensors further comprise steering angle, lateralacceleration and yaw rate.
 5. The system of claim 4, further comprisinga vehicle controllability determination comprising a determination of alongitudinal wheel slip ratio value.
 6. The system of claim 4, furthercomprising a vehicle controllability determination comprising adetermination of a target and an actual tire slip angle.
 7. The systemof claim 4, further comprising a vehicle controllability determinationcomprising a determination of target and an actual yaw rate.
 8. Thesystem of claim 5, wherein the controller reduces regenerative brakingand proportionally increases non-regenerative braking to maintainvehicle controllability when the longitudinal wheel slip ratio value isgreater than 10 percent.
 9. The system of claim 5, wherein thecontroller reduces regenerative braking and proportionally increasesnon-regenerative braking to maintain vehicle controllability when thelongitudinal wheel slip ratio value is greater than 5 percent.
 10. Thesystem of claim 5, wherein the controller reduces regenerative brakingand proportionally increases non-regenerative braking to maintainvehicle controllability when the longitudinal wheel slip ratio value isgreater than a value that is dependent on vehicle operating conditions.11. A method to continuously control braking and controllability of avehicle comprising a generator motor adapted to adjustably provideregenerative braking torque to the wheels of a first axle,non-regenerative brakes being connected to wheels of a second axle, themethod comprising the steps of: providing regenerative braking to wheelsof said first axle; and controlling the vehicle by sensing vehicleconditions including brake position and wheel speed of each wheel,activating non-regenerative and regenerative braking in varyingproportion between wheels of said first and second axles, determiningvehicle controllability based on at least one actual vehiclecontrollability value and at least one predetermined controllabilitytarget value, and reducing regenerative braking while proportionallyincreasing non-regenerative braking to maintain the actual vehiclecontrollability value within the predetermined target value.
 12. Themethod of claim 11, wherein the step of controlling the vehiclecomprises using a simple proportional-integral-derivative feedbackcontroller.
 13. The method of claim 11, wherein said first axlecomprises a rear axle and said second axle comprises a front axle; thenon-regenerative brakes provide braking torque to wheels of the frontaxle; and the step of sensing vehicle conditions further comprises thestep of sensing vehicle conditions for steering angle, lateralacceleration and yaw rate.
 14. The method of claim 11, wherein the stepof determining vehicle controllability comprises determining alongitudinal wheel slip ratio value.
 15. The method of claim 11, whereinthe step of determining vehicle controllability comprises determining atarget and an actual tire slip angle.
 16. The method of claim 11,wherein the step of determining vehicle controllability comprisesdetermining target and an actual yaw rate.
 17. The method of claim 14,wherein the step of reducing regenerative braking while proportionallyincreasing non-regenerative braking is activated when said longitudinalwheel slip ratio is greater than 10 percent.
 18. The method of claim 14,wherein the step of reducing regenerative braking while proportionallyincreasing non-regenerative braking is activated when said longitudinalwheel slip ratio is greater than 5 percent.
 19. The method of claim 14,wherein the step of reducing regenerative braking while proportionallyincreasing non-regenerative braking is activated when said longitudinalwheel slip ratio is greater than a value that is dependent on vehicleoperating conditions.
 20. An automotive vehicle, comprising: a generatormotor adapted to provide regenerative braking torque to the wheels of afirst axle; non-regenerative brakes connected to the wheels of a secondaxle; a controller; and a control system embodied in the controller fordirecting the controller to sense vehicle conditions including brakeposition and wheel speed of each wheel, and, based on said conditions,to activate non-regenerative and regenerative braking in varyingproportion between a front axle and a rear axle, and to determinevehicle controllability based on at least one actual vehiclecontrollability value and at least one predetermined targetcontrollability value, and reduce regenerative braking whileproportionally increasing non-regenerative braking to maintain theactual vehicle controllability value within the predetermined targetvalue.
 21. An article of manufacture to continuously control braking andcontrollability for a vehicle comprising: a generator motor having anability to provide regenerative braking torque to the wheels of at leastone axle; non-regenerative brakes connected to the wheels of at leastone axle; a controller; and a control system embodied within thecontroller for directing the controller to sense vehicle conditionscomprising brake position and wheel speed of each wheel, activatenon-regenerative and regenerative braking in varying proportion betweena front axle and a rear axle, determine vehicle controllability based onat least one actual vehicle controllability value and at least onepredetermined target value, and reduce regenerative braking whileproportionally increasing non-regenerative braking to maintain theactual vehicle controllability value within the predetermined targetvalue.